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Of 707 live\u2010born infants, 15% died within 24\u00a0h, 70% survived \u2265365\u00a0days, 64% were assessed at 2.5\u00a0years. The risk of death within 24\u00a0h after birth decreased with gestational age [odds ratio (<jats:styled-content style=\"fixed-case\">OR<\/jats:styled-content>) 0.3; 95% <jats:styled-content style=\"fixed-case\">CI<\/jats:styled-content> 0.2\u20130.4], antenatal corticosteroids (<jats:styled-content style=\"fixed-case\">OR<\/jats:styled-content> 0.3; 95% <jats:styled-content style=\"fixed-case\">CI<\/jats:styled-content> 0.1\u20130.6), and cesarean section (<jats:styled-content style=\"fixed-case\">OR<\/jats:styled-content> 0.4; 95% <jats:styled-content style=\"fixed-case\">CI<\/jats:styled-content> 0.2\u20130.9); it increased with multiple birth (<jats:styled-content style=\"fixed-case\">OR<\/jats:styled-content> 3.0; 95% <jats:styled-content style=\"fixed-case\">CI<\/jats:styled-content> 1.5\u20136.0), vaginal breech delivery (<jats:styled-content style=\"fixed-case\">OR<\/jats:styled-content> 2.3; 95% <jats:styled-content style=\"fixed-case\">CI<\/jats:styled-content> 1.0\u20135.1), 5\u2010min Apgar score &lt;4 (<jats:styled-content style=\"fixed-case\">OR<\/jats:styled-content> 50.4; 95% <jats:styled-content style=\"fixed-case\">CI<\/jats:styled-content> 28.2\u201390.2), and birth at a level <jats:styled-content style=\"fixed-case\">II<\/jats:styled-content> hospital (<jats:styled-content style=\"fixed-case\">OR<\/jats:styled-content> 2.6; 95% <jats:styled-content style=\"fixed-case\">CI<\/jats:styled-content> 1.2\u20135.3). The risk of death between 1 and 365\u00a0days remained significantly decreased for gestational age and corticosteroids. The risk of mental developmental delay at 2.5 years decreased with gestational age, birthweight and fetal growth; it increased with vaginal breech delivery (<jats:styled-content style=\"fixed-case\">OR<\/jats:styled-content> 2.0; 95% <jats:styled-content style=\"fixed-case\">CI<\/jats:styled-content> 1.2\u20137.4), male gender, low Apgar score and high Clinical Risk Index for Babies score.<\/jats:p><\/jats:sec><jats:sec><jats:title>Conclusion<\/jats:title><jats:p>Several obstetric factors, including abdominal delivery, influenced the risk of death within the first day of life, but not later. Antenatal corticosteroids and gestational age decreased the mortality up to 1\u00a0year. Mental developmental delay was related to vaginal breech delivery.<\/jats:p><\/jats:sec>","DOI":"10.1111\/aogs.12726","type":"journal-article","created":{"date-parts":[[2015,8,6]],"date-time":"2015-08-06T18:42:14Z","timestamp":1438886534000},"page":"1203-1214","update-policy":"https:\/\/doi.org\/10.1002\/crossmark_policy","source":"Crossref","is-referenced-by-count":64,"title":["Impact of obstetric factors on outcome of extremely preterm births in Sweden: prospective population\u2010based observational study (<scp>EXPRESS<\/scp>)"],"prefix":"10.1111","volume":"94","author":[{"given":"Karin","family":"K\u00e4ll\u00e9n","sequence":"first","affiliation":[{"name":"Centre for Reproductive Epidemiology Lund University Lund Sweden"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Fredrik","family":"Serenius","sequence":"additional","affiliation":[{"name":"Women's and Children's Health Section\u00a0for Pediatrics Uppsala University Uppsala Sweden"},{"name":"Department of 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